1. Field of the Invention
The present invention relates to a battery charger and more particularly, to a relatively low cost battery charger which includes a switched regulator, for example, a synchronous buck regulator, formed from discrete components, which eliminates the need for regulator integrated circuit thus reducing the overall cost of the battery charger.
2. Description of the Prior Art
Known battery chargers are normally configured to receive an unregulated source of 120 volt AC input power. Such battery chargers also include a microprocessor for control and a full wave rectifier for converting the input 120 volt AC power to an unregulated source of DC power. Other known battery chargers are known to receive an unregulated source of DC power, for example, 12 volts DC from a vehicular power system.
Battery chargers are known to include a regulator which receives the unregulated source of DC power and convert it to a regulated source of DC voltage. Both linear and switching type regulators are known. Linear regulators utilize linear devices, such as resistors to provide a relatively constant DC output. Such linear regulators may be formed as an integrated circuit (IC). In a typical application, the linear regulator IC provides a generally linear DC output to the battery charger circuit which normally includes a microprocessor for controlling the charging characteristics of the battery charger and either an on-board or external switching circuit, such as a pulse width modulator (PWM) circuit, to selectively connect and disconnect the battery charger and the battery cell under charge. In such an application, the power control is provided by the regulator IC, which, in turn is controlled by the microprocessor. In other words, in such an application, the microprocessor does not directly control the power output level other than switching the circuit on and off.
Commonly owned U.S. Pat. Nos. 5,764,030 and 5,998,966 disclose the use of a discrete PWM circuit under the control of a microprocessor to selectively connect and disconnect the battery charger and the battery cell under charge. These battery charger circuits utilize a separate linear regulator for controlling the output power level of the charger. Such linear regulators are known to have significant power losses due to the resistive components and are thus not suitable in all applications. For example, such linear regulators are not generally suitable for high efficiency applications. As such, switching type regulators are known to be used in battery chargers for such high efficiency applications.
Both buck and boost switching type regulators are known. Boost regulators are used to boost or raise the voltage of the input source of DC voltage. Buck regulators are used to lower the voltage of the input source of DC voltage. Both boost and buck regulators are known to include a primary and a secondary switch, a choke, a transformer and a diode. The primary switch is normally driven by a pulse width modulator (PWM) circuit.
In order to reduce switching losses, synchronous switching type regulators have been developed. An exemplary synchronous regulator is disclosed in U.S. Pat. No. 5,592,071. In such synchronous regulators, the primary and secondary switches are synchronized in order to reduce losses. Indeed, in such synchronous regulators, the primary switch is turned on while the secondary switch is turned off during the first half of each cycle. Similarly, the primary switch is turned off and the secondary switch is turned on during the second half of each cycle.
Various synchronous regulators are known and available as integrated circuits (ICs). For example, Fairchild Semiconductor model Nos. FAN5234 and RC 5057 and Linear Technology model Nos. LTC 1736 and 1709-85 are known. In battery chargers which utilize such synchronous regulators, the ICs normally control the output power level of the battery charger, which, in turn, are under the control of the microprocessor. Such regulator ICs have become the de facto standard in battery chargers. Unfortunately, regulators ICs, such as synchronous regulator ICs, are relatively expensive and add significant cost to the overall cost of a battery charger. Thus, there is a need for a relatively lower cost battery charger which incorporates a switched regulator for increased efficiency.